As the need for increased data storage changes, the search for higher density, faster access memory technologies also increases. One of these, holographic data storage, provides the promise for increased access to higher density data. The techniques for realizing such storage typically utilize some type of storage media, such as photorefractive crystals or photopolymer layers, to store 3-D "stacks" of data in the form of pages of data. Typically, coherent light beams from lasers are utilized to perform the addressing, writing and reading of the data from the storage media by directing these beams at a specific region on the surface of the media. Writing is achieved by remembering the interference pattern formed by these beams at this region. Reading is achieved by detecting a reconstructed light beam as it exits the storage medium, the data then being extracted therefrom. Addressing is achieved by the positioning of the laser beams, and this is typically done through the mechanical movement of mirrors or lenses; however, the storage media itself can be moved relative to fixed laser beams.
There are two types of devices for positioning a data and a reference beam onto a specific location within the holographic storage media, one type for positioning the media itself and one type for positioning the data and reference beam, or a combination of both types. When the media is positioned, this has the advantage of utilizing less complex optics. However, it has some disadvantages in the type of mechanism utilized to position the media in that it is mechanical and thus positioning speed is a concern. In positioning systems that rely upon optics to direct both the data beam and the reference beam, there exists some disadvantages due to the complexity of the optics. For example, if the storage media were dimensioned in a 2".times.2" format, this might require optical lenses on the other of 2"-21/2" in diameter. Further some care must taken in the beam deflection systems utilized in association with an optics-only system to ensure that storage locations on the perimeter of the storage media, i.e., the extrema, are not subject to distortions, as these are probably the most difficult regions to access. Of course, a combination of the two systems could be utilized with the disadvantages of both systems being represented in the combination.
When applying holographic storage techniques to disk-based systems, it is necessary to rotate the storage media to a certain position and either write the information thereto or retrieve the information therefrom. This requires some angular information and radial information as to the location of the storage input region on the disk. Further, this is a two-dimensional storage region and not a one-dimensional storage region. As such, the amount of data that can be stored on the disk is a function of the packing technique utilized wherein the placement of the storage regions is defined. With two-dimensional disks, the storage regions can be organized or arranged such that they are evenly spaced and offset to achieve the most efficient packing density However, this is not readily accomplished on a disk since rows and columns are not present, as would be the case with a rectangular media.